1. Field of the Invention
This invention concerns a method for producing an electrolytic solution for redox batteries, and, in particular, for a highly concentrated vanadium electrolytic solution.
2. Description of the Related Art
In recent years, through global problems such as acid rain, the destruction of the ozone layer by fluorocarbons, and the greenhouse effect due to an increase in carbon dioxide in the atmosphere, environmental problems are being focussed upon as a problem for all mankind. In the midst of this state of affairs, the movement to make the fullest possible use of solar energy, an inexhaustible and clean form of energy that is friendly towards the earth, has increased considerably. Examples of this include solar batteries, power generation making use of solar heat or heat reclamation, wind-turbine generation, and wave power generation (power generation making use of the energy of ocean currents, or temperature differences of seawater).
Of all of these, it is solar batteries, with a remarkable revolution in technology, that show signs of heading towards the time when they are ready for genuine practical applications, through improvements in their efficiency and a significant lowering in price. Currently, use of solar batteries is restricted to rather small-scale applications such as in powering road signs and communications relays, but rapid developments are also expected through envisioned solar energy cities and the implementation of designs to lay fields of batteries in the oceans or deserts. However, the power output of all of these power generation methods making use of solar energy is affected by climactic conditions, thus making stable and trustworthy production of electrical power impossible. Coordinated use of solar energy and reliable, effective batteries are required, and the realization there of has been long awaited.
Moreover, electrical power may be easily converted into other types of energy, is easy to control, and causes no environmental pollution at the time of its consumption, and it is for these reasons that the percentage of total consumption taken up by electrical power is increasing every year. The distinguishing characteristic of electrical power is that its production and consumption are simultaneous with each other, and that it cannot be stored. It is for this reason that, at present, highly efficient nuclear power generation and advanced thermal power generation are being operated at the highest possible efficiency ratings, and that the large increase in demand for electricity during daylight hours is being met by with small-scale thermal and hydropower generation suitable for generating power in response to fluctuations in consumption of electrical power; thus the current state of affairs is such that excess energy is being produced at night. The power generation world is earnestly hoping for development of technology that will make it possible to store this excess energy at night and use it efficiently during the day.
From circumstances such as those above, all types of secondary batteries have been studied as a method of storing electrical energy which does not pollute the environment and as an energy with a wide variety of applications. Redox batteries have received special attention as a high-volume stationary battery capable of operating at room temperatures and atmospheric pressure.
Redox batteries pass an electrically active materials of postive and negative solution to the cells with flow-through electrodes, and making use of a redox reaction, perform charging and discharging of batteries, and thus have a comparatively longer life than normal secondary batteries, with minimized self-discharging, and possess the advantages of being high in both reliability and safety. In recent years the actualization of redox batteries have received considerable attention.
At present, redox batteries which are held to be in the stage of practical use, that is those with redox couple of bivalent and trivalent chromium vs. bivalent and trivalent iron, cannot be made to have cencentrated solutions due to cross-mixing with iron and chromium passing through the membrane of the cells and limitations on solubility. Also, with an output voltage for a single cell of approximately 0.9-1 volts (V), their energy density is low. Furthermore, when the charged state of the electrodes becomes unequal due to generation of hydrogen on the negative electrode, there is the danger of generating chlorine from the positive electrode during charging.
In order to overcome the above faults, the use of a chromium and chlorine redox couple has been proposed (Japanese Patent Laid-open No.61-24172), but in this battery as well the redox electric potential of chromous and chromic ions is close to the electric potential for hydrogen generatin, the problem of a lowering of efficiency due to the generation of hydrogen gas has been solved, and, since it uses chlorine as an active material, there is a problem with storing large quantities of chlorine.
Moreover, as an active material capable of improving the electrode reaction of both postive and negative electrodes, there have also been proposals to make use of iron, copper, tin, nickel, and halogen acidic solutions (Japanese Patent Laid-open No.60-207258), but with any combination of these, problems exist such as the electromotive force for a single battery being low, and the complex electrode reaction of the interaction of metals and the electrodes, so that these solutions were not necessarily satisfactory.
On the other hand, there has also been a proposal for a redox battery with positive and negative electrodes which have trivalent and bivalent ion pairs, and tetravalent and pentavalent vanadium dissolved in a sulfuric solution (U.S. Pat. No. 4,786,567, Journal of Power Sources 15 (1985) 179-190 and 16 (1985) 85-95). This battery has a high output voltage of 1.4 V to 1.5 V, and is characterized by its high efficiency and high energy density, but in order to obtain a high-density vanadium solution, costly vanadyl sulfate must be used, and this has been viewed as being poorly suited for practical use. As a material for the production of the vanadium solution, pentoxide vanadium or ammonium metavanadate are advantageous in price, but the solubility of the former with respect to sulfuric acid solutions is extremely poor and there are also problems with the solubility of the latter, making it difficult to prepare a solution having the concentration necessary for an electrolytic solution, and costly vanadyl sulfate should be used.
Redox batteries are composed of a membrane made up from a ion exchange membrane, carbon cloth electrodes (positive electrode and negative electrode) on either side of this membrane, and an end plate on the outer side of the membrane. The positive electrolyte and negative electrolyte are sent to the positive and negative electrodes from the positive electrolyte tank and the negative electrolyte tank respectively.
In initial charging, on the positive electrode tetravalent vanadium is oxidized into pentavalent vanadium, and in the negative electrode, tetravalent vanadium is reduced to trivalent vanadium. If charging is continued even further, in the negative electrode, trivalent vanadium is further reduced to bivalent vanadium, but on the positive electrode this causes either overcharging or oxygen generation. Therefore, in order to avoid this, it has been necessary the exchange to electrolyte with a tetravalent vanadium solution when the electrolyte on the positive electrode reaches a fully charged state. In this state, when the battery has been fully charged, on the positive electrode, an oxidation reaction from tetravalent to pentavalent vanadium takes place, and on the negative electrode, a reduction reaction from trivalent to bivalent vanadium takes place. In a fully discharged state, the reverse reaction occurs.
Accordingly, charge and discharge reactions are as follows: ##STR1##